1. Field of the Invention
This invention relates to the mounting of windings in transformers, and more particularly to a method for applying compressive pressure to the winding by clamping the winding parts.
2. Prior Art
Windings in transformers are exposed to very great force in the event of a short-circuit, which forces act in a compressing manner in the axial direction of the winding. It is therefore important that the winding in the finished transformer is held under a compressing pressure which is equal to or greater than the contracting force by which the maximum allowable short-circuit current endeavours to contract the winding parts. In this way, the winding is released from its axial support and permanent deformations may arise only when the short-circuit current exceeds said value. In view of this, various attempts have been made to achieve the desired high compressing pressure.
One method comprises prepressing the finished coil-formed winding in a separate press in the winding workshop to the final height of the coil. Thereafter, the coil has been removed from the press pending the mounting onto the transformer core, and the coil has almost resumed its original height when mounting is performed. In view of this, it has been necessary to make use of special press equipment with draw rods. When the windings have been fitted onto the cores of the transformer, this press equipment has been mounted, said equipment consisting of horizontal press beams positioned over the short ends of the windings and joined to draw rods running vertically over the axial length of the windings. These draw rods compress the windings to the desired height, whereafter the upper yoke is interleaved. Finally, the entire press equipment is dismantled after pressure (contraction) has been applied to the upper yoke and the interleaving joints thus have been locked.
Another method has comprised winding tapes of, for example, glass fibre in toroidal loops around the compressed coils. To achieve sufficient strength, however, such an amount of tape has to be wound on, especially in large transformers, that the tapes occupy too much space and the method therefore becomes uneconomical.
Other solutions to the problem are various constructions based on draw rods which are retained even after the coils have been mounted in the transformer. These draw rods, of course, also take up too much space, but in addition, in view of the increased risks of flash-over along the draw rods, it is not suitable to use them in transformers for high voltages.